Aluminum alloy

ABSTRACT

A CASTABLE, AGE-HARDENABLE, ALUMINUM-BASE ALLOY CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, 3.0-4.5 MAGNESIUM, 0.6-1.8 SILICON, 0.4-1.5 SILVER ZIRCONIUM AT A CONCENTRATION EFFECTIVE TO IMPART INCREASED FAIGUE STRENGTH TO THE RESULTANT ALLOY, AND THE BALANCE ALUMINUM.

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*IU SSENOHVH H. D. BEWLEY ALUMINUM ALLOY Oct. 17, 1972 3 Sheets-Sheet 3 Filed Dec. 9, 1971 m If 2026228 no ZQSZE 2 3.552; 55;: 3 63 mm. 2 m I I I l I I l l I l l L m o+xv x$- I: .ll I 0N (ISd ooon ssaaus United States Patent Oflice 3,698,890 Patented Oct. 17, 1972 Int. Cl. C22c 21/02 US. Cl. 75-147 4 Claims ABSTRACT OF THE DISCLOSURE A castable, age-hardenable, aluminum-base alloy consisting essentially of, in weight percent, 3.0-4.5 magnesium, 0.6-1.8 silicon, 0.4-1.5 silver zirconium at a concentration effective to impart increased fatigue strength to the resultant alloy, and the balance aluminum.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 63,457, filed Aug. 13, 1970.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

The present invention relates to an improved aluminumbase alloy. More particularly, it relates to a castable, corrosion-resistant, age-hardenable, aluminum-base alloy having improved fatigue properties.

The basic process for the separation of the isotopes of uranium by gaseous diffusion centers around the pumping of uranium hexafluoride through a barrier and maintaining a proper pressure differential across the barrier. This is accomplished with axial-flow compressors driven by an electric motor. The axial-flow compressor consists mainly of a rotor, stator, blades, and casing. The rotor is a cylindrical drum with a shaft along its axis, with several rows of blades attached around the periphery of the rotor extending transversely to the rotor axis. The stator is generally cone shaped and sits just outside the tips of the rotor blades defining an annular opening between rotor and stator from the suction end to the discharge end. Blades similar to the rotor blades are positioned in rows inside the stator. As the rotor turns, the gas to be compressed is drawn from the suction end, with each row of blades building up the gas pressure as the gas flows from suction to discharge end. Because of the combination of corrosion resistance, castability, and high strength-to-weight ratio, aluminum alloys have been used as the material of choice for compressor components, especially compressor blades. One of the best alloys used for this purpose is identified herein as the reference 214X having a composition consisting essentially of, in weight percent, 3 to 4.5 magnesium, 0.6 to 1.8 silicon, and the balance aluminum. This alloy is readily castable into compressor blades, exhibits moderate tensile properties (less than 30,000 psi. ultimate tensile strength) and fairly high fatigue properties (about 13,000 p.s.i. fatigue limit). As compressors are driven to higher power levels, more stringent demands are placed on compressor components,

especially compressor blading. Compressor tests at high power levels show that the endurance limit or fatigue strength of the 214X alloy is exceeded resulting, in turn, in extensive deblading and damage to other compressor components.

SUMMARY OF THE INVENTION It is, accordingly, the main object of this invention to provide a new and improved alloy having all of the desirable properties of the aforementioned reference 214X alloy, but with a heightened level of mechanical properties, especially fatigue strength.

This invention is based on the discovery that small additions of silver and silver with zirconium within specified limits lead to a hard, strong alloy capable, with appropriate age-hardening treatment, to operate at heretofore unobtainable endurance limits. In accordance with the present invention, it has been found that small additions of silver, from as little as 0.4 to as much as 1.5 percent by weight, lead to appreciable improvement in the mechanical properties of the 214X alloy. Silver in this range of concentration has been found effective to modify the aging process in a manner which involves interaction with diffusion-controlling vacancies. Both vacancy and solute diffusion to sinks are restrained by the presence of silver in the concentrations used in this invention. Electron microscopy studies have shown that silver in effective amounts changes the 214X-type alloy by producing a fine, evenly dispersed, precipitated phase as opposed to a segregated structure where any precipitated phase is bunched or associated mainly vw'th grain boundaries. Thus, the ad dition of silver within prescribed limits results in an appreciable increase in hardness and tensile strength relative to the reference 214X alloy. Most of the improvement in hardness and tensile strength is achieved by additions of silver in the range 0.4 to 1.0 silver. Additions of silver up to 1.5 percent result in slight additional improvement and must be balanced against the cost of the additional silver.

When additions of zirconium are combined with silver, the fatigue strength or endurance limit of the resultant alloy is far beyond that known for any similar aluminummagnesium-silicon alloy as represented by the reference 214X system. Addition of zirconium up to 0.5 percent combined with the aforementioned silver addition leads to some improvement in hardness and tensile strength and, most importantly, to a dramatic increase in endurance limit or fatigue strength. Concentrations beyond 0.5 percent tend toward development of a brittle secondary phase.

The drawings of FIGS. 1-3 are graphs in which FIG. 1 shows the effect of silver and zirconium alloying additions on the age hardening response at 350 F.

FIG. 2 shows the effect of silver on age hardening induced at 350 C. relative to the reference 214X alloy.

FIG. 3 shows the improvement in fatigue properties obtained by silver and zirconium additions in an alloy aged to full hardness.

Improved structural stability of the new alloy of this invention is brought about by a combination of compositional as well as structural modification. This means that the addition of silver and/ or zirconium alone to the reference alloy will not, of itself, lead to the improved result. It must, in addition, undergo an aging process, by which is meant that the composition-modified alloy must be heated to and held at a temperature which leads to the development of a secondary precipitated phase as characterized by electron microscopy. The development of this age-hardened condition is achieved by maintaining the silveror silver-and-zirconium-modified alloy at a temperature in the range 300-350 F. for a period of time sufiicient to produce a desired maximum hardness. The development of a maximum age-hardening condition does not require a prior solution treatment but rather can be obtained from as-cast material. The alloy is relatively insensitive to the cooling rate from the melt and both die-cast and permanent mold-cast parts respond well to the aging treatment. The age-hardening or precipitate-inducing temperature is 300-350 F. Lower temperatures may be used, but the development of a maximum hardness level takes an inordinately long time. Higher temperatures than 350 F. up to as much as 500 F. can be used, but are not desirable because maximum hardness levels and associated strength properties are reduced.

Alloys within the scope of this invention can be melted and held in standard cast iron pots. The silver can be added in the metallic form and dissolution of even relatively large ingots is rapid. Experience has shown that losses of silver from the melt are negligible even after a remelt operation. The zirconium can be added either in the form of a zirconium-containing flux or as a standard high-zirconium aluminum-base hardener. Additions by using the hardener are preferred. Zirconium melt losses may occur; so periodic compositional checks and new zirconium additions are needed to insure maintenance of the proper level of zirconium. Small permanent mold castings containing varying amounts of silver and zirconium to modify the reference 214X composition were produced from each resulting alloy to provide age-hardening, tensile, and fatigue test bars.

AGE-HARDENING EFFECT Age-hardening tests were conducted on various cast specimens to test the effect of composition and prior fabrication history on the maximum achievable Rockwell Hardness. The effect of alloying additions on the agehardening response of as-cast 214X reference alloy at 350 F. is shown in FIG. 1, which is a plot of hardness as a function of aging time at 350 F. It is seen from FIG. 1 that appreciable increments of hardness can be imparted to the reference 214X alloy by the addition of 0.9 percent silver and that further increments of hardness can be imparted by including small amounts of zirconium.

Although, as stated previously, the silver-modified alloys do not require a solution treatment to respond to aging, a comparison of the modified alloys with 214X after such a treatment (8 hours at 960 F. followed by a water quench) reveals that the addition of silver will effect the age-hardening response. Aging tests conducted on a 214X and 214X+Ag alloy after a solution treatment have showed the solution-treated hardness of all the alloys to be somewhat below their as-cast values and that the 214X alloy without silver addition exhibited little response to an aging treatment. However, the 214X+0.9 Ag alloy responded to reach a maximum hardness of 73 Rockwell Hardness in only 15 hours; the 214X+0.4 Ag alloy was intermediate in its response. The age-hardening behavior of solution-treated alloys is shown in FIG. 2.

It should be noted that the maximum hardness achieved at the age-hardening temperature will be maintained in the alloy so long as it is operated at temperatures below that used to introduce the age-hardening effect. Thus, a plot of hardness-versus-temperature at any temperature below about 50 F. of the age-hardening temperature will show a constant hardness over an extended time.

TENSILE TESTING Tensile tests were conducted at room temperature on the alloys of this invention in both the as-cast and full- 4 hard (as-cast and aged to full hardness at 350 F.) conditions. The results are shown in Table I below.

TABLE I Elonga- TS, YS, tion, Alloy k s.i. k s.i. percent As-cast 214x 22. 1 17. 1 2 214X+0.9 Ag 29. 8 22. 4 3 214X+0.9 Ag+0.2 Zr".-. 31.8 21. 9 3

As-cast plus aged to full hardness at 350 F.

FATIGUE TESTING Fatigue bars were machined from sections of heattreated permanent mold castings and tested in a standard, fully reversed, bending, rotating beam fatigue machine. All tests were conducted at room temperature.

The endurance limits of several representative alloys within the scope of this invention are presented in Table II below.

TABLE II Endurance Temperature limit, Alloy condition p.s.i.

14X As-cast 13, 000 214X+0.9 Ag.--" A ,000 214X+0.9 Ag 1 B- 14,000 214X+0.9 Ag+0.2 Zr A--- 12,000 214X+0.9 Ag+0.2 Zr--- B 19,000

As-cast plus aged to full hardness at 350 F. 9 As-cast plus aged (236 hours at 350 F.) The data in Table II demonstrate the remarkable improvement in endurance limit achieved by chemical and physical modification of the base of the reference 214X alloy; namely, by the addition of silver and zironium combined with an aging treatment at temperature to produce a finely dispersed array of small age-hardening particles. As Table II shows, the endurance limit of silvermodified alloys subjected to an aging treatment at 350 F. was raised to 14,000 p.s.i. as compared to a silvermodified alloy without the aging treatment. The most dramatic improvement is shown with aged alloys containing zirconium. In that case the endurance limit was raised to 19,000 p.s.i., representing a 46 percent increase over the endurance limit of the as-cast 214X alloy. S/N curves (permissible stress/number of cycles before failure) are shown in FIG. 3.

As used in the specification the term consisting essentially of refers to essential elements of the alloy-to elements which are deliberately mixed to form the desired alloy having a desired combination of properties. It should, however, be understood that small amounts of other elements may be part of the alloy as claimed which are not deliberately added but which appear in the final alloy in the process of its manufacture. Thus, such impurities as copper up to 0.12 percent, chromium up to 0.1 percent, and iron up to 0.45 percent may be tolerated without adversely influencing the desirable qualities of the alloy. Iron may be tolerated even up to amounts of from 0.45 to 1.8 percent provided there is deliberately added 0.45 to 0.7 percent manganese at a Mn/Feratio suflicient to 5 7 convert the iron-containing phase to a rounded nodular shape as opposed to elongated platelets.

The presence of iron in amounts greater than 0.45 percent up to 1.8 percent results in a secondary iron-containing phase in a form which reduces the mechanical properties of the alloy. Addition of manganese averts the adverse etfects of iron by converting into a form which does not adversely affect the mecahnical properties of the alloy. In that case, a modified form of the alloy of this invention consists essentially of, in weight percent, 3 to 4.5 percent magnesium, 0.6 to 1.8 percent silicon, 0.4 to 1.5 percent silver, zirconium at a fatigue-strengthening concentration, preferably in the range 0.2 to 0.5 percent, 0.45 to 1.8 percent iron, 0.45 to 0.7 percent manganese, and the balance aluminum.

What is claimed is:

1. A castable, age-hardenable, aluminum-base alloy consisting essentially of, in weight percent, 3.0-4.5 magnesium, 0.6-1.8 silicon, 0.4-1.5 silver zirconium at a concentration eflective to impart increased fatigue strength to the resultant alloy, 0.45 to 1.8 iron, 0.45-0.7 manganese, and the balance aluminum.

. 6 2. The alloy of claim 1 wherein the zirconium concentration in the alloy is in the range 0.2-0.5 weight percent. 3. As a new article of manufacture, a compressor blade casting of an alloy consisting essentially of, in weight percent, 3.0-4.5 magnesium, 0.6-1.8 silicon, 0.4-1.5 silver, zirconium at a concentration effective to impart increased fatigue strength to the resultant alloy, 0.45-1.8 iron, 0.45- 0.7 manganese, and the balance aluminum, said alloy being in an age-hardened condition by heat treatment at a temperature above the intended service temperature.

4. The article of claim 3 wherein the zirconium concentration in the alloy in the range 0.2-0.5 weight percent.

References Cited UNITED STATES PATENTS 3,306,717 2/1967 Lindstrand et a1 75-147 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

